1. Field of the Invention
The present invention relates to an optical low-pass filter suitable for use with an optical apparatus having an image pickup element, such as a video camera or a digital camera.
2. Description of Related Art
If an object having a periodical structure whose frequency is higher than a frequency corresponding to the pixel period of an image pickup element such as a CCD is to be photographed by using an optical apparatus having such an image pickup element, for example, a video camera or a digital camera, the image pickup element will output a false signal or a false color and the phenomenon of a degradation of a photographed image occurs.
To prevent this phenomenon, it is known to employ an optical low-pass filter which separates an image of an object into two or more images and cuts the high-frequency component of the image by making use of the double refraction of a crystal plate.
If a sufficient low-pass effect is to be achieved by such optical low-pass filter, the optical low-pass filter needs to have at least two crystal plates. However, the use of expensive crystal plates leads to the problem of an increase in cost. Furthermore, since the action of crystal does not have an appropriate effect on a polarization type of object, the problem of a decrease in the low-pass effect will occur.
To cope with these problems, Japanese Patent Publication No. Sho 44-1155 discloses an optical low-pass filter which separates a wavefront into two or more images by means of a plurality of prisms.
However, the optical low-pass filter disclosed in Japanese Patent Publication No. Sho 44-1155 still has a problem. As shown in FIG. 1, the optical low-pass filter can separate an image into two images on an image plane IS1, but there is another image plane IS2 on which the two images are mixed into one image. In an optical apparatus provided with an AF mechanism which determines the state of focus on the basis of the state of an image plane, the AF mechanism determines the image plane IS2 as an in-focus position, with the result that the low-pass effect decreases.